From what i could tell in the pics, the right side bracket and backing plate are bolted through that east street panel. If that panel was welded in id say no problem but you are really just relying on those few rivets to hold that panel in which i dont think are adequate. Not to mention your harness eyelet is on the seat bracket which is a bad idea and relying on those rivets as well. I have mounted probably over a hundred miata seats and i rarely have to remove the stock front mounting bracket, i think it really strengthens the floor.
I like those east street panels and im not knocking your work, i just think seats need to be strongly secured for the belts to serve their purpose.
Jedd, thanks for your input. One of the reasons I posted was to give people some options to consider and discuss. As we build our cars, there are lots of decisions and compromises, all with pros and cons.
I don't disagree with your comments, but please understand two things:
1) the two stud plates are not individual, stand alone fasteners. On each side they are 2' long.
2) Nothing is solely bolted to the riveted tunnel panel. The load is spread over the entire 2', which, on the right side, is mostly a combination of the stock floor board and the tunnel panel.
Admittedly, I can't provide numbers to prove what force it would take to rip the mounts out of the car, but it would be substantial. I'm convinced these stud strips are stronger than four individual fasteners (even with 4"x4" backing plates).
I agree that leaving the cross brace in would strengthen the floor but I felt it was a worthwhile compromise.
Lastly, I'm not thrilled with my right side lap belt attachment, but it is stronger than if I had attached the eyebolt into the thinner tunnel sheet metal. You can't see it in the pictures, but there is some reinforcement that helps put some of the force in-line (tension rather than just shear).